The present invention relates to a plant for the biological depuration of air polluted by odorous compounds, organic and inorganic, particularly for the depuration of air in which the content of polluting odorous substances is rather low but strongly troublesome from the environmental point of view and also the flow rate of the air to be treated is not very high.
In the following specification reference shall be made to odorous substances, mainly meaning substances having bad smell, but it is not to be construed in a limiting sense.
As a matter of fact, the present invention can be successfully applied to the removal of organic volatile substances (O.V.S.), also chlorinated.
It is well known that the pollution of air with odorous substances occurs not only as a consequence of the emissions of heavy industries, such as petrochemical, textile, food and agricultural, paper manufacturing, tanning, fertilising and like industries, but also owing to other industrial activities such as the cattle breeding, slaughtering and mainly the treatment plants for solid garbage and the treatment of emissions of plants for the depuration of town and industrial effluent waters.
A typical case is that of the reclamation of polluted lands (for example by solvents) by means of well known operations (sieving, treatments with soil conditioning compounds, etc.), pursuant to which the surrounding air is polluted with rather low concentrations of polluting substances.
For the depuration of polluted air several technical approaches have been proposed and developed up to industrial use, which can be classified into two main lines, namely the prevention of the pollution (trying to eliminate or at least reduce the contents of polluting substances in the air) and the treatment of the already polluted air.
In the latter case worth of mention is the treatment based on the so-called synergetic biotechnology essentially involving the use of a filtering bed of an inert macroporous adsorbing support, the pores of which contains micro-organisms able to carry out the degradation of the odorous substances, adsorbed by having the polluted air flowing through the filtering bed.
The adsorbing support is coated with a water repellent layer and within the pores optimum conditions (such as humidity, nutrients, temperature and the like) are established for promoting the activity of the said micro-organisms.
This technology is disclosed in the European Patents Nos. 414,095 and 497,214, the content of which is herein recalled for a more detailed information.
A substantial advantage of the treatment process according to this technology is that the polluting substances are destroyed by the action of the micro-organisms and anyhow from the treatment only inert residues are obtained (mainly inorganic salts, water and carbon dioxide) the disposal of which does not need further treatments.
A further advantage of the process of the aforesaid European patents is that it includes, downstream of the above mentioned filtering bed, a further adsorbing and/or absorbing step, wherein polluting substances, not yet removed from the air and destroyed by the action of the micro-organisms, are retained by being adsorbed and subsequently recycled to the treatment in said filtering bed during the dead times of the primary depuration process.
A problem exists in the case of the depuration treatment of air polluted by odorous substances when small flow rates of air are involved (up to 10,000 to 12,000 S.cu.m./hour), since the known plants, independently from the technology upon which they are based, even in the minimum size, are certainly oversized and consequently too much expensive, as regards both the building thereof (which means high investment costs) and the normal operating costs, so that often the problem is put aside.
Typical cases are those of the small sewage treatment plants, collecting pools or lagoons in swine breeding, small paper manufacturing plants, textile dyeing plants, plants for lifting industrial and civil sewages, and the like.
To date this specific technology, characterised by very small operating costs and for the almost total absence of non inert by-products, has not found possible use in the treatment of modest flow rates of polluted air.
The main aim of the present invention is that of providing a plant by which the treatment of polluted air for the depuration from odorous volatile substances in the case small flow rates of air is carried out in an effective and industrially advantageous.
A more specific aim of the present invention is that of providing a plant in form of self-contained and autonomous unit by which polluted air is admitted through a main inlet and automatically undergoes a full depuration, thus coming to the outlet in a condition proper for being returned to the atmosphere, the polluting substances being destroyed or anyhow converted to inert compounds which are disposed without any further problem or need of treatment.
These and other aims are achieved by a the plant in form of autonomous and self-sufficient unit consisting of:
(a) a prescrubbing section, in which the air to be treated is brought to a desired content of water, in particular saturated with water, the scrubbing water being possibly added with chemicals suitable to adjust the pH of the water as a function of the polluting substances contained in the air whereby the scrubbing takes place in conditions close to neutrality or slightly acid;
(b) a biological filter, the air to be depurated flowing through it from the top to the bottom, comprising a bed of macroporous inert particles, the pores of which contain a population of micro-organisms capable of biologically destroying the chemical substances contained in the air passing through the filter, the surface of the filtering bed being preferably coated with an adsorbing substance of hydrophobic nature, the surface of the filtering bed being sprayed in a controlled way with nutrient substances and other possible additives as a solution or dispersion in water, and
(c) a postscrubbing section comprising a scrubbing chamber wherein the air undergoes a final washing with water under oxidising conditions, the water being possibly added with suitable compounds suitable to promote such an oxidising effect,
the air flow through the plant being caused by a drawing apparatus placed downstream of the postscrubbing section.
In the preferred embodiment of the plant according to the present invention the above components are housed in a compact unitary container-like framework, easy to install and to run, whereby only one inlet for the air to be depurated and only one outlet for the depurated air are provided, the latter being preferably in form of a chimney, the plant further comprising automatic monitoring devices of the air entering the several sections, so as to constantly control the nature and the amounts of chemical additives added to the water fed to the several sections.
Preferably, upstream of the prescrubbing section there is placed a unit for heating the air to be treated, which is brought to a controlled temperature, suitable for promoting the optimum activity conditions of the microorganisms which are present in the biological filter.
Furthermore, an adsorbing bed might also be placed, upstream of the postscrubbing section; said adsorbing bed can be activated in order to remove from the air coming from the biological filter possible polluting substances not demolished or anyhow not separated in the biological filter, for their subsequent elimination, thus preventing not perfectly depurated air from being released in the atmosphere.
In particular, said adsorbing bed can be connected to the prescrubbing section by using, as a vehicle for collecting and recycling the polluting substances, an air flow going through the adsorbing bed itself.
In such a way, the present plant is able to cope also with temporary overloads of polluting substances in the air to be depurated.
Moreover the water fed to the several sections is recycled in the sense of being returned by merely falling back into the tanks from which it is drawn. From the above definition it is clear that the plant of the present invention achieves the results sought for in industrially advantageous manner especially for the treatment of modest flow rates of air polluted by bad smelling substances.